These machines which are rapid-operating are usually fed from stock by automatic feed means. The automatic feed means brings the parts one by one to a point adjoining the zone where the part is processed in the machine and a transfer arm is then operated to bring the part from this point to the processing zone in the machine. Similarly a transfer arm may be provided to remove the part from the processing zone to an automatic discharge device. Known transfer arms are usually endowed with a complicated motion comprising a to-and-fro horizontal motion and two identical vertical motions comprising a raising and a lowering of the arm when the arm has finished its horizontal motion in the forwards direction and in the return direction. The horizontal motion may be a motion of rotation about a vertical axis or a motion of translation of the arm in the horizontal plane. The horizontal motion serves to transfer the part from the automatic feed device to the machine or from the machine to the device for automatic discharge of the parts. The vertical motions enable picking up and laying down of the part from the machine feed device or from the machine and on the discharge device respectively.
For picking up or releasing the parts, these transfer arms are equipped at their ends with gripping devices which may be tongs, suckers or magnetic devices. These gripping devices may be actuated once at the moment of picking up the part and once at the moment of laying the part down.
The transfer arms are, for their movement, attached by the end opposite that which carries the gripping device, to a drive device which executes the horizontal and vertical motions composing the complex motion required. These devices must fulfill a certain number of conditions necessitated by the conditions of use of the transfer arms in association with rapid-operating machines. Thus the motions of the transfer arm must be perfectly synchronized with the different stages of the working cycle of the machine when it is required to allow the machine to work continuously, that is to say, without stopping the manufacturing cycle at the time of bringing up or of discharging the parts moving the transfer arm. In the majority of cases it is in fact quite desirable to operate continuously in order to make better use of equipment which is often costly. Additionally continuous running of machines, particularly presses, reduces the wear on members such as clutches which are actuated only rarely. Even in the case where operation is stroke by stroke it is desirable to reduce the transfer time and hence the time of stoppage of the machine. So in all cases it is desirable that the transfer time be as short as possible and in the case of continuous running rates of the order of 100 to 200 cycles per minute may be desirable. By transfer cycle is to be understood the whole of the operations comprising picking up a part on the feed device, conveying it and putting it down on the processing machine and the return of the arm to its starting position above the feed device. Obviously an equivalent definition can be given in the case of a transfer arm employed for discharge of machined parts. Another feature required of the transfer arm in all cases of its employment is accuracy of positioning. That is, in order that picking up of the part and conveyance of this part to the required spot in the machine be effected correctly it is necessary for the arm to have movements which are closely defined and not liable to variation with time. It is in short inconceivable to have to carry out frequent adjustments on devices for movement of transfer arms operating in association with highspeed machines carrying out series manufacture.
The devices employed hitherto do not permit very high speeds to be attained nor perfectly synchronized and very accurate movements of the transfer arm to be obtained. Devices for transfer arm movement are known which, for example, effect the horizontal and vertical motions of the arm by a roller rolling between a cam and a counter-cam with a very small clearance necessary for reducing friction. This clearance causes a lack of accuracy in the motions which gets steadily more pronounced with wear, in service. On the other hand each of the rollers controlling the movement of the arm horizontally and vertically is connected to a driving member attached to the transfer arm by a group of gears and relatively long levers having moderate rigidity and constituting masses in varying motion the inertia of which limits the performance both from the point of view of the speed and of the accuracy of the motions of the transfer arm.